The present invention relates to a method of analyzing signals received by elastic wave pickups for automatically locating, with high precision, significant instants such as the time of first arrival at the various pickups and/or the end times of these signals.
U.S. Pat. Nos. 4,775,009, 5,303,773, and 5,370,545 notably describe various techniques for monitoring the evolution in time of underground reservoirs using seismic or other pickups permanently installed in one or more wells, without disturbing the various operations in progress (fluid production or injection, various servicing operations carried out by means of these wells). Permanent installation of these pickups in wells allows seismic monitoring of a reservoir in order to detect various phenomena linked with the development thereof. Permanent seismic pickups are for example installed outside a casing that is lowered into the well. They are embedded in the cement that is normally injected into the annular space between the casing and the well and which provides proper coupling with the surrounding formations. The seismic pickups can also be fastened outside a string installed in a well during completion operations. They are associated with mobile devices suited to press them against the well casing and with decoupling devices for filtering the vibrations propagated along the string.
French Patents 2,703,457, 2,703,470 and 2,728,973 and U.S. Pat. No. 5,724,311 describe methods intended for long-term repetitive active monitoring of a reservoir by applying elastic waves to a formation and acquisition of the response signals reflected by the formation, by means of permanent installation of emission-reception devices in wells or in the vicinity of the ground surface. Differential processings are thus performed on acquisitions obtained under identical conditions.
French Patent 2,772,137 describes a method which simplifies problems of discrimination and identification of seismic or microseismic events relative to the development of an underground zone. One or more reference pickups are used, which are provided with a direct acoustic coupling with elements of the technical zone development equipment that can also detect waves directly linked with development. Comparative analysis of the signals coming from the various receivers and from each reference pickup allows sorting of the records into different families according to whether the events in the underground zone are independent of the events detected by each reference pickup or depend thereon directly or indirectly. A catalog of type events having a causal connection with development operations in the underground zone can be drawn up by correlation between the records of the signals received respectively by the receivers and the (or each) reference pickup, and the nature of the seismic or microseismic events can thus be identified.
Precise determination of the times of arrival of the signals received by several receivers coupled with the underground formation allows location of the points of emission (source points or origins) where events directly or indirectly are linked with an activity that modifies the stress field such as fluid injection or extraction operations through one or more wells.
This allows the site operator to quickly interpret the position data obtained in connection with the conventional production data (pressure, flow rate, temperature, etc.), so the operator can take into account the mechanical response of the site in the production protocol so as to preserve the productivity of the well(s) or of the site.
However, the most commonly used method for precisely detecting the significant instants of the signals received during seismic monitoring of an underground zone is the operator viewing all the records and locating the times (of first arrival) when the signals occur on all the pickups. Such an operation is long and tedious when large numbers of systematic records are obtained over a long period of time which require review.
The method according to the invention, which provides analysis of acquired signals coming from at least one wave pickup, allows automatic detection with high precision at least one significant instant of each signal such as the time of first arrival (ti) or the end time (tf) thereof which simplifies counting and sorting operations prior to interpretation.
The method of the invention has applications in many fields where the times of arrival of vibrational signals are to be located with high precision. This is notably the case within the scope of systems intended for active or passive seismic monitoring of underground reservoirs under development, generally used for fluid extraction or for fluid injection.
The method comprises:
determining the pseudoperiod corresponding to an inverse of the dominant frequency of the frequency spectrum of at least part of each acquired signal;
coding each acquired signal so as to form a binary coded signal in reference to a first threshold value depending on the acquired signal (coding to value 1 or 0 for example, according to whether its absolute value is above or below this threshold value);
forming for each acquired signal an integral signal by stacking the binary coded signal on a sliding window so as to determine a second threshold value (preferably depending on the maximum amplitude of this integral signal); and
positioning a significant instant of an acquired signal in reference to an instant where a corresponding integral signal reaches a second threshold value.
In order to position the significant instant (ti or tf) of each acquired signal, a first reference point is for example determined, where the amplitude of the corresponding integral signal is equal to the second threshold value, a first intermediate instant between the first reference point and a boundary that is the closest to the integral signal (the beginning, if the time of first arrival is sought) is detected, a second intermediate instant between the first intermediate instant and an opposite boundary of the integral signal (the end, with the same hypothesis) where the integral signal exceeds a third threshold value depending on the acquired signal is also detected. The significant instant sought is then located on the acquired signal at a quarter of a pseudoperiod of the second intermediate instant (upstream or downstream according to whether it is the time of first arrival or the end time of each acquired signal).
The time of first arrival of each acquired signal and the end time of each acquired signal can both be determined and parts recorded between these two instants can be isolated in the records of the signals received by each wave pickup.
All the recorded parts defined by various significant instants can also be isolated on a single acquired signal so as to detect all distinct arrivals at the same wave pickup.
The method can also comprise analyzing the signals respectively received by a plurality of wave pickups, relative to an event generating elastic signals, determining polarization of each acquired signal immediately after a time of first arrival and detecting successive arrivals of compressional waves and shear waves by comparing orientations of the eigenvectors linked with the respective polarizations of the signals.
The method can also comprise analyzing the acquired signals coming from a plurality of wave pickups, relative to an event generating these signals, determining polarization of each acquired signal immediately after a time of first arrival and detecting multiplets by comparing orientations of the eigenvectors linked with the respective polarizations of the signals.
The method according to the invention allows automatic detection of the arrival of a (notably seismic) wave with a very good precision. The method is therefore self-adaptive because it reaches its objective: evaluation of the dominant frequency of each acquired signal and use of the pseudoperiod corresponding to the dominant frequency to investigate an integral signal (binary coded and stacked) established from the unprocessed initial signal, thus taking into account a series of several parameters adjustable according to the local signal acquisition conditions, but depending on the signal itself.
The method therefore facilitates automatic location of the microseismic events having a succession of compressional and shear waves and, if these events are detected by a sufficient number of spaced out pickups, the method allows automatic triangulation from the times of arrival thus determined, the velocity model being known otherwise.